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With the decreasing of finish rolling temperature, the microstructure of the hot rolled steel was changed from lath martensite to complex microstructures composed of lath martensite, ferrite, granular bainite and lath bainite, unrecrystallized prior austenite grains are the main reason for the formation of complex microstructures. After tempering, the steels are mainly composed of ferrite and cementite. For the steel with high finish rolling temperature, the proportion of high misorientation boundary is less, and it has rod-like precipitates with the length of 25−85 nm and punctate precipitates with the size of 4−16 nm, the intensities of {111} and {001} textures are enhanced. The experimental steel has the best combination with strength and toughness as yield strength of 996 MPa, tensile strength of 1023 MPa, total elongation of 19.1 %, impact value at -60 ℃ of 125 J. Grain refinement, higher density of dislocation, fine and dispersed precipitates could enhance the yield strength and tensile strength. Stronger intensity of {332} texture and weaker intensity of {001} texture could improve the impact toughness.

The microstructure and properties of a Fe-2Cr-Mo-0.12C (wt pct)-tempered steel plate at different normalization temperatures after hot rolling were investigated to enhance its comprehensive mechanical properties. The microstructure of the steel was mainly composed of a ferrite-bainite-tempered martensite mixture after the normalization-tempering process. The tensile properties and the ductile-brittle transition temperature values indicated that the best comprehensive mechanical properties can be obtained by normalizing at 930 °C and tempering at 700 °C. When the normalization temperature was 880 °C to 930 °C, the yield strength increased as the bainite and tempered martensite hard phase volume fractions increased. However, when the normalization temperature surpassed 980 °C, the precipitation strength behavior compensated for the adverse influence of the effective grain size growth, and yield strength was nearly unchanged. Total elongation reached a peak value when normalized at 930 °C, due to the optimal volume fraction ratio of the soft to hard phase. The Charpy impact fracture changed from a ductile to a brittle fracture with an increase in the normalization temperature. When the normalization temperature was 930 °C, the existence of a certain volume fraction of nano-precipitates and soft ferrite in the microstructure contributed to excellent ductile properties.

In this study the strain capacity and work-hardening behavior of bainite (B), bainite + polygonal ferrite (B + PF), and bainite + polygonal ferrite + pearlite (B + PF + P) microstructures are compared. The work hardening exponent (n), instantaneous work hardening value (ni), and differential Crussard-Jaoul (DC-J) analysis were used to analyze the deformation behavior. The best comprehensive mechanical properties were obtained by the introduction of the pearlite phase in B + PF dualphase with the tensile strength of 586 MPa and total elongation of 31.0%. The additional pearlite phase adjusted the strain distribution, which increased the initial work hardening exponent and then maintained the entire plastic deformation at a high level, thus delayed necking. The introduction of pearlite reduced the risk of micro-void initiation combined with the high frequency of high angle grain boundaries (HAGBs) in triple-phase steel, which led to a low crack propagation rate.

Microstructure and mechanical properties of the Fe-1.2Mn-0.3Cr-1.4Ni-0.4Mo-C tempered steel plate in three cooling processes (direct quenching, accelerated cooling + air cooling and air cooling) after hot rolling were investigated in this paper. The lath-martensite is obtained by direct quenching, whereas granular bainite, lath bainite, retained austenite and M/A constituents are observed in other two cooling processes. Meanwhile, the lath-martensite with higher cooling rate after hot-rolling could enhance the intensity of {332} texture, which is beneficial for the strength and toughness of the steel. After tempering at 680°C, the difference of yield strength is mainly caused by grain boundary strengthening and precipitation strengthening. The higher proportion of high misorientation boundary, stronger intensity of {332} texture, weaker intensity of {001} texture and fewer of M/A constituents (or retained austenite) will improve the impact toughness. All in all, the steel with optimum combination of strength and toughness is obtained by using direct quenching to room temperature after hot rolling.

In-situ tensile test accompanied by electron backscatter diffraction (EBSD) analyses were performed on polygonal ferrite (PF) and bainite dual-phase steel, selected regions of interest were analyzed following plastic deformation of the steel. Deformation-induced crystal orientation evolution, localized strain concentration, slip transfer, and geometrically necessary dislocation (GND) density were tracked. Results revealed that heterogeneity deformation facilitated formation subregions with crystal orientation deviation in grain and fragmented the grain by the new low angle grain boundaries (LAGBs) or medium angle grain boundaries (MAGBs). The PF grains with ND// preferred crystal orientation exhibited high orientation stability, and almost all load axes of the selected PF grains moved to the [101] pole, resulting in enhancing {111} orientation component at high strain levels. With the lattice rotation during deformation, the high angle grain boundaries (HAGBs) can change to MAGBs, which was beneficial to maintain coordination deformation among grains. Localized strain concentration can be decreased by the slip transfer across the PF grain boundaries or bainite/PF phase boundaries, which reduced the risk of micro-void formation. Additionally, the variation of α12 GND tensor average value (Ave. α12) revealed that the ferrite was continuous plastic deformation, while the bainite occurred stage hardening. The required strain for the coordination deformation was controlled by strain hardening behavior.

Thermal simulated specimens with the heat inputs of 20, 50 and 80 kJ/cm were used to investigate the effects of heat input on the microstructure and mechanical properties of the Fe–2Cr–Mo–0.12C pre...

Micro-textures and mechanical properties of Fe–21Cr–15Ni–6Mn–Mo–Nb austenitic stainless steel during asymmetric hot rolling were studied. The results show that the shear texture is the dominant texture on the upper surface of steel sheet when the specific velocity ratio is 1.1, and the orientation of γ-Fiber was beneficial to recrystallization and nucleation. The texture strength of the central layer of the steel plate was concentrated in the Cube orientation. When the velocity ratio increases to 1.2, the complete recrystallization of the upper and central layers of the plate was more likely to occur under the condition of high deformation energy, and the structure of the steel sheet had tended to homogenize. Under the conditions of low reduction (20%) or low finishing temperature (860 °C), the steel sheet was partially recrystallized or non-recrystallized, and there were a lot of deformation textures during rolling. In the process of asymmetric hot rolling, the crystal rotates around the direction of TD, and the initial texture of weak γ-Fiber and Copper deviates from the ideal position and gradually approaches to the direction of γ-Fiber. Based on the research, the tested austenitic stainless steel had high strength and toughness (the yield strength, the tensile strength and the elongation is 710.1 MPa, 1010.2 MPa and 46.7%) under the rolling process with specific speed ratio 1.2, the initial rolling temperature 1100 °C and the reduction 60%.

The work-hardening characteristics and deformation behavior of high deformability pipeline steel with a polygonal ferrite and bainite microstructure were studied in this paper. The high work-hardening ability of polygonal ferrite with 35% bainite (PF+35%B) dual phase sample contributed to its high tensile properties. Electron backscattered diffraction was conducted to analyze the mechanism of the micro-damage behavior. The results of kernel average misorientation and grain reference orientation deviation of the deformation microstructure revealed that the main deformation mechanism of the polygonal ferrite grain was slip with a number of slip systems. Dislocation rearrangement and a local orientation gradient caused by crystal rotation during sliding to cause the formation of micro-voids. The strain dispersion ability of the PF+35%B sample was better than that of the polygonal ferrite with 27% bainite (PF+27%B) dual phase sample. The risk of micro-voids nucleation and crack propagation were reduced to an improved plastic deformation ability for PF+35%B sample. The average Taylor factor (M) of the PF+35%B sample was higher than that of the PF+27%B sample, which indicated the grains of the PF+27%B sample was more liable to slip during deformation and yield under low stress. The evolution characterization of the micro-texture showed that the γ-fiber was more stable compared with the α-fiber, and the stronger γ-fiber may contribute to the improved plasticity.